The invention relates to a device for the analysis or absorption measurement of a small quantity of a liquid medium, for example, a drop, by means of light, which is guided through the medium and which then can be detected or analyzed photometrically, spectrophotometrically, fluorometrically, or spectrofluorometrically. The device has a receiving point area for depositing or applying drops of the medium at the top in the position of use, a horizontally oriented light inlet located underneath the receiving point in the position of use in its housing, and a first device located behind the light inlet in the beam path for guiding the light upwards towards the receiving point.
In many cases, only small quantities of liquid samples are available. This applies primarily in biochemical, medical, or pharmaceutical analysis. For systematic surveys with a plurality of similar samples, for example, titration plates as sample containers are used, which allow efficient, sequential, or parallel analysis.
For individual measurements for qualitatively very different substances and especially for absorption measurements, such samples are filled into cells and then analyzed, in a known way.
For cells with a measurement chamber volume of ten microliters and less—that is, approximately drop sized—due to the small substance quantity, it is difficult to introduce the sample of the liquid medium into the measurement channel, because suitable cells provide only small access openings and the measurement channel has a correspondingly small cross section. When introducing a sample of the medium to be analyzed into the measurement channel, it must be guaranteed that the channel is completely filled by the medium and, for example, that there are no bubbles within the volume of the liquid medium passed through by the measurement beam, because bubbles could considerably falsify the measurement results.
Therefore, cells are usually first filled outside of a corresponding measurement device and the quality of the filling is checked visually. Then the cell is inserted into the cell holder of a measurement device. Here, the cell must be positioned in the measurement beam, so that there is no variation in the absorption level in the cell itself due to the screening effect of the measurement chamber cross section, because such variation could also falsify the measurement results. This can occur because the beam cross section of a spectrophotometer used as the measurement device usually exceeds the free aperture of the mentioned cell with very small sample volumes. Even small deviations in the positioning of the cell in comparison with a reference measurement or through the use of several cells, which are structurally identical but subject to variation in manufacturing, within one measurement cycle can lead to deviation in the measurement values. After the measurement, due to the shape of the cell holder in commercially available measurement devices it is practically unavoidable to be required to remove the cells from the holder for emptying and cleaning.
From DE 33 44 387 A1 a photometer head for small measurement volumes is known, in which a pair of light transmitters and light receivers is arranged in a material block and this material block has a recess, which is covered with a plate, in the area of facing surfaces of the light transmitter and light receiver. This plate has a bore, through which an application device can be guided with a spacing between the facing surfaces of the light transmitter and light receiver. Therefore, the use of a cell is to be avoided. The drop of the liquid medium to be analyzed is to be introduced into this spacing and must be held there despite the force of gravity acting on it. Thus, the application of a sample drop must be carried out with great care, so that this drop is held stationary in the spacing, which is open at the bottom, between the surfaces despite the force of gravity, which incidentally also requires a corresponding consistency of the medium to be analyzed.